So. I did several tests.
I've edited settings in Kodi : Preferences -> Services -> SMB Client with "Minimum supported protocol = SMBv2, Maximum supported protocol = SMBv3".
Kodi can't see my macOS server (in Files -> add a source -> smb, there's nothing).
If I set "Minimum supported protocol" to SMBv1, it can see my macOS server. When I add a new source, I can see "WORKGROUP -> macOS". But when I select "macOS", it still answers "Operation not permitted".
I tried too to write in Kodi my administrator user name and password with no result. I'd be OK to create a "LibreELEC" user on macOS with full credentials, to give it access to my shared videos but it doesn't work.
When booted on Ubuntu, it's now OK even with "Minimum supported protocol = SMBv1" and "Maximum supported protocol = SMBv3" (first time I though I had to set also "Maximum supported protocol" to SMBv1).
EDIT : I have found another option in Kodi. When I set "Maximuum supported protocol" to SMBv1, I can check "Use legacy security" (I don't know what does it mean). With these settings, when I try to connect to my macOS server, it answers "Connection time out"...
It's not better... but different 